The present invention relates to computer-assisted radiological measurements on radiographic images.
In musculoskeletal radiology, it is current practice to use a wide range of imaging modalities to determine skeletal disorders and abnormalities.
In this discipline diagnosis is often based on quantified radiological findings of geometrical quantities.
At least 50% of all radiological examinations today are conventional exposures of thorax and the skeleton. 80% of skeletal exposures leads to the correct diagnosis based on the radiographs.
A plurality of textbooks in the X-ray diagnostics from world-renowned radiological and orthopaedic experts make a contribution in this area.
However, to refine the diagnosis, to conduct better differential diagnosis, to assess severity of change, to plan and control therapy, to conduct treatment follow-up, to establish physical ability in sports medicine, labour medicine and military medicine, verbal descriptions based on skeletal radiographs are insufficient in many respects.
Better diagnosis can be achieved by quantifying radiological findings.
Geometrical quantities measured on radiological images must be checked against normal values. These normal values have been collected from measurements of a representative sample of the normal healthy population and are tabulated in the above-mentioned text books.
Geometrical measurements in digital images comprise linear and angular measurements. Linear measurements in 2 dimensions and 3 dimensions may be supplemented with distance along a curvilinear path. Angular measurements are considered in the plane of the image, in a world plane or in 3D space. Geometrical areas are considered in the image plane, or more generally of surface patches in 3D images. Volumes are computed in 3D images but may be based on planar measurements. Geometrical indices are clinical quantities based on image measurements. In any of these categories, the measurand is defined as the physical parameter being quantified by measurement.
Today, radiological measurements on X-ray images are either made on film using conventional measuring devices (such as a ruler, a caliper or a rubber band to measure lengths, and a square or goniometer to measure angles) or in a digital image displayed on screen using cursor controlled points (such as a pair of points to measure Euclidean distance between).
The current measurement procedure thus involves 4 distinct media:
1. An X-ray film comprising the anatomical sites to be measured, displayed on a light box. With the emergence of digital radiography modalities (film digitisation, computed radiography, digital radiography sensors), the digital image may be displayed on a computer display. However, such electronic medium still is physically different from the other components described hereafter.
2. A measurement atlas comprising the measurement scheme: imaging technique, graphical template and description of the measurements covered by the scheme (nomenclature, clinical significance, and normative tables, sometimes interchangeably represented by curves)
3. An analogue measurement device (ruler, square . . . ) to perform geometrical measurements,
4. Pencil/Paper to note the measurement quantity according to the appropriate medical nomenclature and the measurement value.
A calculator device may be needed to compute indices from a collection of measurements, or to convert measured values to true quantities using calibration measures. Alternatively, electronic spreadsheets may be used in conjunction with a database to store the measurements and indices.
The use of different media asks for repeated focusing of attention between the atlas and the radiological image.
Moreover, in the absence of an atlas scheme the position of the measurement objects is not defined and hence different users may locate a given anatomical landmark differently.
Because there is no link between the measurement template in the atlas and its associated measurement entities, there is no systematically imposed consistency of the naming of measured quantities, and therefore exchange or collection of measurement values of different clinicians (e.g. for the purpose of cross-refereeing) is fundamentally hampered.
Another major drawback of the prior art method to perform geometrical measurements is increased measurement error or measurement uncertainty.
The error of measurement is the result of a measurement value minus the (true) value of the measurand. Measurement error is due to different sources, basically falling into one of two classes: systematic and random errors.
Systematic or bias errors arise from consistent and repeatable sources of error (like an offset in calibration).
Systematic errors can be studied through inter-comparisons, calibrations, and error propagation from estimated systematic uncertainties in the sensors used. Systematic error is defined as the mean that would result from an infinite number of measurements of the same measurand carried out under repeatability conditions minus the (true) value of the measurand. This source of error can be reduced by better equipment and by calibration.
Random errors also referred to as statistical errors, arise from random fluctuations in the measurements. In particular, digitisation noise (e.g. geometric digitisation: finite pixel size; intensity digitisation: quantisation of grey levels) and the errors introduced by counting finite number of events (e.g. X-ray photon count) are examples of random errors in the context of digital X-ray images. Random error is defined as the result of a measurement minus the measurement that would result from an infinite number of measurements of the same measurand carried out under repeatability conditions. Particularly this source of error is prevailing in the prior art of performing measurements on X-ray images.
Inter-observer and intra-observer variance on measurement values contribute to this source of error, and has its origin in several forms of ambiguity in defining the measurand.
Lack of unambiguous definition of the measurand with respect to the imaged patient anatomy and lack of knowledge of the geometrical pose of the patient with respect to source and detector are the main source of random error.
Repeatability and reproducibility of a measurement require that the random errors involved in the measurement procedure are low. Although random errors are reduced when a measurement is repeated many times and the results averaged together, this can rarely be achieved in clinical practice.
It is an object of the present invention to provide a user-friendly radiological measurement method that overcomes the drawbacks of the prior art.
The above-mentioned objects are achieved by a method as set out in claim 1.
The method of the present invention is described with regard to geometrical measurements performed on radiological images of humans. It will be clear that the invention is not limited to human beings and can also be applied in other fields, for example in veterinary applications.
In the context of the present application the term xe2x80x98activationxe2x80x99 refers to loading a measurement scheme from memory and measurements to be performed according to the loaded scheme.
A measurement scheme or measurement template is a pattern of measurements to be performed. The measurements to be performed are grouped in the form of a measurement procedure wherein the sequence, the inter-dependence and method of measurements are defined. Such a measurement scheme can be noted and stored in a computer in standard notation XML (Extensible Mark-up Language).
In general the measurement scheme comprises a graphical part (also called graphical model) and an internal part (also called internal model).
The graphical part represents the geometric relation between measurement entities (objects and operators) and the anatomy in the type of image on which the measurements are to be performed. Measurement objects are e.g. points, lines, circles etc. The measurement objects are defined relative to the anatomy. The intended position with respect to the anatomy is thereby unambiguously denoted.
The measurement objects can be labelled e.g. with the appropriate medical nomenclature. Other kinds of naming are possible, e.g. for naming intermediate objects needed in the course of the measurement procedure.
Measurement entities also comprise distances, angles which are the result of the measurement procedure and which are preferably also named according to medical convention.
The graphical part may also comprise measurement mark up such as a graphical indication of distance or angle.
It may also comprise textual annotations of the names of objects and entities.
In the graphical part, a region of interest box around a set of measurement points may be superimposed, to indicate the area to which geometric operators (such as zooming) or intensity operators (such as enhancement or landmark extraction operators) can be applied.
To ensure correct match with the image, e.g. with the imaged patient body part, the correct exposure parameters can be specified in the template, quantifying the imaging geometry. Angle of incidence and source-patient-detector distances are the most important geometric parameters, which much be observed at exposure time for a specific template to be applicable. Specific data structures and methods of each measurement entity and object class are devoted to control their graphical content and behaviour.
The measurement scheme may also comprise normative values associated with the measurement entities. These normative values are e.g. organised as a normative value table and are used for comparison with measured entities. Abnormal values may be signalled.
The internal part represents the functional dependencies between measurement entities. Functional dependencies comprise type of measurements, measurement methods, location of a measurement entity in a measurement dependency graph, order in which measurements are to be performed etc.
The internal model can be implemented as an object-oriented model of a coherent set of measurements to be performed on a digital (medical) image.
It can for example be represented by a measurement dependency graph. The internal model decomposes each measurement of a physical quantity into its constituent measurement objects. It further specifies methods to map the objects geometrically onto the medical image.
In informatics terms, the model is composed of objects belonging to an associated class, consisting of a collection of data structures and methods, said methods operating on the corresponding data structures.
The flow of execution of the measurements of the measurement scheme is imposed by a measurement dependency graph, in which the nodes correspond to measurement of geometric objects, and directed arcs define the relationship between the nodes.
As will be explained further on, the internal part may activate the nodes in several ways:
sequential point-only operation will first address all measurement points prior to evaluating the measurement object and operator nodes;
the sequential measurement object operation will let fire each node immediately when the results of all children are available, representing the pure data-flow case;
a node representing a set of points linked to an active contour model will invoke an automatic segmentation computation in order to compute the location of the image mapped points prior to firing the nodes representing geometrical objects based on said mapped points.
The graphical part and the internal part of the measurement scheme are bi-directionally linked such that the master-slave relation between the two parts is defined. The bi-directional link provides that measurements can be activated starting from the graphical part as well as starting from the internal part.
This means that
either internal methods of the graphical objects may be invoked by addressing the graphical content of the measurement scheme. After pointing to a geometric entity in the stencil, a method, appropriate for the type and location of the geometric entity, is invoked to map the entity in the image. For example, clicking on a line may invoke the user interface method to position two pairs of juxtaposed points, each point pair yielding a midpoint coincident on the requested line. In contrast to a passive bitmap display of the measurement objects, which would require a knowledgeable user to map them graphically, the measurement scheme supplies the appropriate method. Analogously, a complete sub-graph corresponding to a measurement operator in the measurement dependency graph may be invoked from the measurement template window. For example, when the mouse cursor is over a the graphical representation of a measurement operator in the template window, it may change from an arrowhead appearance to a measurement operator symbol (e.g. a distance or an angle symbol), denoting that the associated sub-graph in the measurement dependency graph may be activated by pressing the mouse cursor.
or methods affecting the graphical behaviour of the measurement methods in the stencil and the radiological image may be steered by the internal methods or the internal control flow (e.g. a graphical entity in the stencil highlights when the corresponding node in the measurement dependency graph is visited).
In one embodiment a measurement scheme is selected from a measurement stencils repository. A measurement stencils repository comprises a number of measurement stencils. The selection is preferably performed in correspondence with the examination type and/or the radiographic exposure conditions.
The measurement scheme can be implemented as an active or as a passive measurement scheme.
An active measurement scheme (also called measurement template) is differentiated from a passive one in that the measurement procedure is guided in part or completely by a computerised procedure operating on image data. Guidance may range from simple computer guided sequencing of measurement operations to fully automatic execution based on automatically determining landmarks and deforming model contours representing anatomy.
The graphical part of the measurement scheme can be implemented as an stencil-overlay on the displayed X-ray image. The points and lines of the scheme must then be dragged by the user to the correct anatomical position in the displayed image.
In one embodiment, display of the graphical part of the measurement scheme may be omitted when measurements are simple and performed routinely on the radiograph. User guidance is effectuated here solely through the measurement dependency graph, which may generate the measurement points and objects immediately in the image, after which the user is requested to map them to their actual position. An example of such simple measurement is the cardio-thoracic index, which ratio is calculated routinely on thorax AP radiographs.
In one embodiment, more than one radiographic image may be displayed and a measurement scheme may be activated with more than two associated graphical parts. This configuration is useful for performing 3D measurements from measurements on a limited number of projections. For example, 3D spinal measurements may be obtained from locations of anatomical points identified on frontal and lateral radiographs of the spine. A measurement template with two graphical parts is activated here; one for each of the projections, and each graphical part guides the user in the mapping of the projection point of a certain 3D point in the associated projection images. A measurement point node in the measurement dependency graph will compute its 3D coordinates from the locations of the point in the respective projections. Likewise, 3D measurement object and operator nodes have 3D methods to calculate the 3D parameters of the object and the 3D measurement values respectively from the values of their children nodes.
Specific features and further embodiments of the present invention are set out in the dependent claims.
One aspect of the present invention relates to a computer program product adapted to carry out the method of the present invention when run on a computer. The computer program product is commonly stored in a computer readable carrier medium such as a CD-ROM. Alternatively the computer program product takes the form of an electric signal and can be communicated to a user through electronic communication.
The method of the present invention is advantageous over the prior art for the following reasons.
A first advantage of using a measurement template is the unambiguous and instantaneous indication as to the position of the measurement objects in the actual image. That is, the first source of random error in the prior art, due to a badly or non-defined measurand, is eradicated.
In a film-based method or in a generic computerised measurement method in conjunction with a radiological measurement atlas, repeated focusing of attention back and forth between the atlas and the radiological image are needed to locate the points precisely.
xe2x80x83Moreover, in the absence of an atlas scheme, the position of measurement objects is not defined and hence different users may locate a given anatomical landmark differently.
xe2x80x83In accordance with the present invention, the use of a digital template, especially when displayed in proximity of the radiographic image, substantially reduces the effort to locate measurement object points precisely and hence reduces intra-observer measurement error (error in repeated measurements of the same quantity performed by a single user) and inter-observer measurement error (error in repeated measurements of the same quantity between different users).
In the present invention the measurements are digital in contrast to the analogue measurements of the film-based method The precision of positional location is thus dependent on the image resolution (physical size of a pixel unit, which typically is around 100 xcexcm for digital detectors).
In a specific embodiment measurement errors may be further reduced by the possibility of zooming into portions of a radiograph prior to locating a measurement point, and occasionally applying a sub-pixel location algorithm to define the position of image points with precision up to a fraction of a pixel unit.
Interesting measurement quantities are often derived from basic measurements by calculations that combine many measurements into final quantities, or from transformations, filters and fit procedures applied on the measurements. In such cases the uncertainty characteristics of the derived quantities can be derived by error propagation.
The ensembles of digital measurements laid out in a measurement scheme collectively define a standard manner on how the procedure must be performed. Standardisation is advantageous because it enables objective comparison (e.g. in second opinion gathering).
By the use of a measurement scheme the order of placement of measurement objects in the image is fixed and logical (eventually the order may be customised within the constraint of dependency). Therefore, in the template-guided measurement method of the present invention, there is no need to study the measurement scheme in an atlas to detect the order in which to place the measurement objects so as to guarantee that all depending objects are placed first. This way of operation is fundamentally different from the film-based method, where measurement templates may even be absent altogether. Repeatability of measurement procedures, obtained by an imposed order, enhances throughput of diagnostic evaluation of the radiograph, which is important for clinical departments such as emergency radiology and intensive care units.
The automatic abnormal value signalling functionality provided by a specific embodiment of the method of this invention, operating instantaneously on measurement values as they become available, makes the necessity superfluous of having the normative reference tables at hand, such as published in the open literature and measurement atlases.
The method of the current invention is particularly suited to implement different measurement schemes in the field of paediatric radiology, since measurement templates can be customised to reflect changes in paediatric anatomy over time. The graphical part of a measurement template will display the measurement points, objects and entities using anatomical landmarks corresponding to the patient""s age. Furthermore age-specific normative values associated with the measurement template can be applied.
In the field of orthopaedics the invention is particularly useful because instantiated measurement schemes can be stored and used for further follow-up after therapy. The graphical part of an older template can be retrieved from memory and can serve as a new template for a current measurement. The older measurements can be compared with the new measurement results to assess evolution of a patient""s condition. No confusion as to the manner in which the measurement objects and entities were defined is possible. Furthermore, second opinion gathering becomes more objective when it is based on the same graphically defined measurement scheme.
The method of the present invention is particularly useful in emergency radiology since the measurement scheme relies on the placement of a set of measurement point only after which the auto-completion of all dependent measurements is implied and automated so that the procedure lends itself to quick execution. This feature is not available in the prior art based on manual measurement based on analogue film measurements or computer measurements with a generic measurement tool.
As it will be clear from the description in the sequel, both repeatability and reproducibility are substantially enhanced by the stencil-guided measurement method of the present invention.
Repeatability conditions include (a) the same measurement procedure, (b) the same observer, (c) the same measurement instrument, used under the same conditions, (d) the same location and (e) repetition over a short period of time.
Whereas fulfilment of these conditions is not guaranteed by the prior art methods, conditions (a), (c) and (d) are met by the current invention because use of a stencil and programmed methods to define the measurands ensure an identical procedure applied under all circumstances. Conditions (b) and (e) are fulfilled because the instantiated measurands in the image according to the stencil are stored and supplied to other referring clinicians, who only need to perform a confirming analysis.
A valid statement of reproducibility requires specification of the conditions changed. The changed conditions may include: (a) principle of measurement, (b) method of measurement, (c) observer, (d) measuring instrument, (e) reference standard, (f) location, (g) conditions of use, (h) time. Whereas fulfilment is not guaranteed by prior art film-based methods and generic computerised measuring tools, the stencil-based method as laid out in the current invention enhances reproducibility considerably. In particular, reproducibility conditions (a), (b), (d), (e), (g) are met because the definition of the measurands and their determination are fixed by and laid out in the stencil. The stencil-based method of the present invention is further invariant to reproducibility conditions (f) and (h), and condition (c) is achieved because storage and retrieval of prior instantiated measurement schemes render the need to re-perform the measurement scheme superfluous.
The objective of a measurement is to determine the value of the measurand, that is, the value of the particular quantity to be measured. A measurement therefore must begin with an appropriate specification of the measurand, the method of measurement, and the measurement procedure. In general, the result of the measurement is only an approximation or estimate of the value of the measurand and thus is complete only when accompanied by a statement of the uncertainty of that estimate.
Prior art methods such as film-based methods and generic computerised measurements do not cope with the problem that neither measurands nor measurement method are defined.
As a result, the uncertainty of the estimated value of a measurand cannot be determined because (a) a measurand must be defined by a standard method of measurement and (b) the implementation of the standard measurement method.
In the stencil-guided measurement method of the present invention, both definition of the measurands and implementation of the measurement method are prescribed.
Specific embodiments of the present invention will be explained with reference to the following drawings.